Refugium

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A refugium is an appendage to a marine, brackish, or freshwater fish tank in which water flows in, baffles slow it down to allow a certain amount of stagnation, and flows back out into the tank. Sizes vary from as small as a shoe box to as large as an entire aquarium. Inside the semi-stagnant portion, various suppositories can be placed to allow biological filtration to take place. The items depend on the type of tank, but common items include crabs, snails, or other creatures that will break down solid waste, or certain algae or live plant species to reduce nitrate and phosphate levels. Confining the plants and creatures to the refugium allows the tank to remain aesthetically pleasing. Using natural creatures is more conducive to life in the aquarium than adding chemicals, since many chemical additives produces side effects.